United Way Worldwide, the world's largest charity, has launched a center to combat modern-day slavery and is pushing to make the eradication of human trafficking a priority for the next U.S. president.

Mara Vanderslice Kelly, the center's executive director, said she wants every U.S. presidential candidate to answer the question of how much money they would put towards ending trafficking.

While there are an impressive number of organizations working to stop slavery, they are vastly under-resourced, something that must change to make a real difference, she said.

"If traffickers are making $150 billion a year in profits, how much are we going to spend to make it a fair fight?" Kelly said in an interview.

Experts estimate more than 21 million people worldwide are victims of human trafficking, with many working in the sex industry, as domestic help, on farms, in fisheries and construction sites.

But the true scale is unknown, and the Walk Free Foundation estimates that 35.8 million people are trapped in modern-day slavery, including bonded and child labor.

The United States spends less than $150 million a year on anti-trafficking efforts, which Kelly described as a drop in the bucket.

"We want to make this a pivotal issue in the U.S. presidential campaign," she said.

U.S. President Barack Obama in 2012 called ending human trafficking "one of the great human rights causes of our time" and put White House resources behind the issue. The 2016 U.S. presidential race is just starting to heat up, with more than a dozen candidates so far and more expected to declare.

The Center on Human Trafficking and Slavery, based at United Way Worldwide's headquarters in Alexandria, Virginia, outside the nation's capital, will campaign to put trafficking on the political agenda, pressure for more funds, and support anti-trafficking efforts by its affiliate groups around the world.

The United Way has 2.6 million volunteers and works in nearly 1,800 communities in more than 40 countries, and raises about $5 billion in charitable donations each year.

Kelly said the center also will work collaboratively with other anti-slavery groups such as Washington-based Polaris, which runs national human trafficking hotlines, and youth organizations such as the YMCA and Boys and Girls Clubs to find the most effective ways of addressing trafficking.

As a start, the United Way announced it will try to buy only products and services that are slave free, by using technology developed by the organization Made in a Free World to assess the risk of slavery within the business supply chain.

"By doing this, United Way is truly walking the walk and setting a good example for other organizations, public and private, to follow to reduce their slavery footprint," Mira Sorvino, the actress who is U.N. goodwill ambassador to combat modern-day slavery, said in a news release.